The effects of elk on aspen in the winter range in Rocky Mountain National Park

National parks face problems in managing wild ungulates and their forage resources, including small park sizes and incomplete protection of winter ranges, absence of major predators, and influences from exterior management activities. Our study focuses on the effects of elk Cervus elaphus browsing on aspen Populus tremuloides in the elk winter range of Rocky Mountain National Park. Elk can prevent successful regeneration of aspen by suckers, and increase the mortality of established trees. Here we quantify the effect of elk on aspen in the elk winter range of the park, particularly since the adoption of a policy of ''natural regulation'' in 1968. We collected a stratified-random sample of aspen cohorts in the elk winter range, and also inside two long-term exclosures. In each cohort we tallied live and dead trees by diameter, recorded the height and percentage of stem surface covered by bark-stripping, and estimated the date of regeneration of the cohort using increment cores. We also tallied live and dead aspen suckers (young trees <2.5 m tall) by height and the number of live and dead branches on each sucker. Aspen outside the exclosures are declining as evidenced by: 1) high mortality among established trees, including entire stands that are dead, 2) low density of live suckers, insufficient to regenerate the stand, 3) suckers that are excessively branched and seldom surviving to reach tree height, and 4) almost no cohort regeneration since adoption of natural regulation. Exclosures and early photos of the elk winter range suggest these are anomalous conditions. Over the last century, aspen cohorts regenerated only when there were fewer than ca 600 elk on the park's elk winter range, far fewer than the present estimated 1600 head. Fire suppression, natural succession, fluctuations in beaver populations, climatic fluctuations, and other factors are of no or lesser importance than elk in the regeneration and survival of aspen cohorts on the elk winter range. The decline of aspen is a concern when management is focused on the ecosystem scale rather than simply the scale of wildlife and their primary forage resources.